The proposed work is in five areas: 1) The development of methods for culture of megakaryocyte colonies in vitro and the required methods for their identification. 2) The development of bioassay techniques for detecting thrombopoietin. 3) The simultaneous measurement of platelet turnover with several available techniques in patients with thrombocytopenia. 4) We will attempt to clarify the platelet function defect which develops during the course of cardiopulmonary bypass. We believe that this is an example of the refractory state. 5) We are developing a radioimmunoassay for antiplatelet antibody.